1. Field of the Invention
This invention relates to a process for upgrading residual petroleum oils to more valuable products, and more particularly to a process wherein hydrogen deficient residual petroleum oils are thermally cracked in the presence of a hydrogen donor diluent.
2. Description of the Prior Art
It is known in the art to upgrade hydrogen deficient residual petroleum oils (resid) by thermally cracking the resid in admixture with a hydrogen donor diluent. The hydrogen donor diluent is a material, generally aromatic-napthenic in nature, that has the ability to take up hydrogen under mild hydrogenation conditions and to readily release the hydrogen to a hydrogen deficient resid under thermal cracking conditions. One of the principal advantages of the hydrogen donor diluent cracking (HDDC) process is that it can upgrade resids which are not readily amenable to other conversion processes, and another principal advantage is that it can provide high conversions in the absence of a catalyst and with a minimum of coke deposition. The cracked materials produced by the HDDC process are readily recovered as desirable products including light ends and a gasoline fraction, and the hydrogen donor diluent can be recovered by fractionation of the cracked products and recycled through the hydrogenation step for reuse as donor diluent in the cracking unit.
The HDDC process is well known in the art, and a comprehensive description of the process, including materials, flows and operating conditions, appears in U.S. Pat. No. 2,953,513. Variations of the HDDC process, particularly as to the make-up of the hydrogen donor diluent, are described in U.S. Pat. Nos. 2,873,245 and 3,238,118. Hydrogen donors proposed in the prior art include relatively low boiling, pure, and expensive compounds such as naphthalene, tetralin, decalin, anthracene, and the like. These compounds have generally been considered unsatisfactory for a commercial operation because of their expense and other difficulties inherent in their use. More practical hydrogen donor diluents suggested by the prior art include certain distillate thermal tars which, upon partial hydrogenation, produce a good hydrogen donor material. However, even when distillate thermal tars are utilized as a hydrogen donor diluent, the hydrogen utilization, yields and selectivities have been such that the hydrogen donor diluent cracking process has been a marginal operation, and the process has not been extensively used commercially. A particular problem encountered using known hydrogen donor diluents is the tendency of the diluent to form coke in the cracking unit, and there has accordingly been a need for a more thermally stable hydrogen donor diluent.